Use $::processorcount to compute API workers number

As we do in other OpenStack Puppet modules, let's use $::processorcount
fact to compute the number of API workers and improve scalability for
cinder-api service.

Change-Id: Iac3e1305ba0b22d70e874e038eb10914311d5695
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>